Antique weighing scales work by using the scale's center of gravity and the equilibrium of the components. A pointer indicates the scale is at zero. The object to be weighed is placed in one pan, then standard weights are added to the other pan until they establish equilibrium.
Equal arm beam scales, the most common type, consist of a uniform beam that is suspended at its exact center by a fulcrum. The fulcrum is like a knife-edged hinge, set at right angles, and has a pointer attached to it. When there is nothing in the scales, or the object being weighed matches the standard weights, the pointer points to zero. Two pans on chains are suspended from the beam each at an equal distance from the fulcrum.
The fulcrum is a crucial part of the scale. It is where the center of gravity is located, but it also helps to determine the sensitivity of the scale. If the fulcrum has more friction, the scale is less accurate. Other factors that play into the scale's accuracy are the total mass of the set of scales and the length of the beam.
Mechanical scales of this type are now considered collector's items and have been out of use since the mid-20th century. Many of these scales are made of brass, were hand-crafted and are considered works of art.